A single-cell wireless LAN using the IEEE 802.11 Wireless LAN Standard is a Basic Service Set (BSS) network. When all of the stations in the BSS are mobile stations and there is no connection to a wired network, it is an independent BSS (IBSS). An IBSS has an optional backbone network and consists of at least two wireless stations. A multiple-cell wireless LAN using the IEEE 802.11 Wireless LAN Standard is an Extended Service Set (ESS) network. An ESS satisfies the needs of large coverage networks of arbitrary size and complexity.
The IEEE 802.11 Wireless LAN Standard is published in three parts as IEEE 802.11-1999, IEEE 802.11a-1999, and IEEE 802.11b-1999, which are available from the IEEE, Inc. web site http://grouper.ieee.org/groups/802/11. The IEEE 802.11 Wireless LAN Standard defines at least two different physical (PHY) specifications and one common medium access control (MAC) specification. The IEEE 802.11(a) Standard is designed to operate in unlicensed portions of the radio spectrum, usually either in the 2.4 GHz Industrial, Scientific, and Medical (ISM) band or the 5 GHz Unlicensed-National Information Infrastructure (U-NII) band. It uses orthogonal frequency division multiplexing (OFDM) to deliver up to 54 Mbps data rates. The IEEE 802.11(b) Standard is designed for the 2.4 GHz ISM band and uses direct sequence spread spectrum (DSSS) to deliver up to 11 Mbps data rates.
Other wireless LAN standards include: Open Air (which was the first wireless LAN standard), HomeRF (designed specifically for the home networking market), and HiperLAN/2 (the European counterpart to the “American” 802.11a standard). Bluetooth is a personal area network (PAN) standard. It is aimed at the market of low-power, short-range, wireless connections used for remote control, cordless voice telephone communications, and close-proximity synchronization communications for wireless PDAs/hand-held PCs and mobile phones.
The IEEE 802.11 Wireless LAN Standard describes two major components, the mobile station and the fixed access point (AP). IEEE 802.11 networks can also have an independent configuration where the mobile stations communicate directly with one another, without support from a fixed access point. The medium access control (MAC) protocol regulates access to the RF physical link. The MAC provides a basic access mechanism with clear channel assessment, channel synchronization, and collision avoidance using the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) access method. The MAC provides link setup, data fragmentation, authentication, encryption, and power management.
Synchronization is the process of the stations in an IEEE 802.11 wireless LAN cell getting in step with each other, so that reliable communication is possible. The MAC provides the synchronization mechanism to allow support of physical layers that make use of frequency hopping or other time-based mechanisms where the parameters of the physical layer change with time. The process involves sending a beacon packet to announce the presence of a wireless LAN cell and inquiring to find a wireless LAN cell. Once a wireless LAN cell is found, a station joins the wireless LAN cell. This process is entirely distributed in wireless LAN cells and relies on a common timebase provided by a timer synchronization function (TSF). The TSF maintains a 64-bit timer running at 1 MHz and updated by information from other stations. When a station begins operation, it resets the timer to zero. The timer may be updated by information received in a beacon packet.
In an independent BSS (IBSS) wireless LAN cell, there is no access point (AP) to act as the central time source for the wireless LAN cell. In a wireless LAN cell, the timer synchronization mechanism is completely distributed among the mobile stations of the wireless LAN cell. Since there is no AP, the mobile station that starts the wireless LAN cell will begin by resetting its TSF timer to zero and transmitting a beacon packet, choosing a beacon period. This establishes the basic beaconing process for this wireless LAN cell. After the wireless LAN cell has been established, each station in the wireless LAN cell will attempt to send a beacon after the target beacon transmission time (TBTT) arrives. To minimize actual collisions of the transmitted beacon frames on the medium, each station in the wireless LAN cell will choose a random delay value, which it will allow to expire before it attempts its beacon transmission.
In order for a mobile station to communicate with other mobile stations in a wireless LAN cell, it must first find the stations. The process of finding another station is by inquiry. The inquiring may be either passive or active. Passive inquiry involves only listening for IEEE 802.11 traffic. Active inquiry requires the inquiring station to transmit and invoke responses from IEEE 802.11 stations.
Active inquiry allows an IEEE 802.11 mobile station to find a wireless LAN cell while minimizing the time spent inquiring. The station does this by actively transmitting queries that invoke responses from stations in a wireless LAN cell. In an active inquiry, the mobile station will move to a channel and transmit a probe request frame. If there is a wireless LAN cell on the channel that matches the service set identity (SSID) in the probe request frame, the responding station in that wireless LAN cell will respond by sending a probe response frame to the inquiring station. This probe response includes the information necessary for the inquiring station to extract a description of the wireless LAN cell. The inquiring station will also process any other received probe response and beacon frames. Once the inquiring station has processed any responses, or has decided there will be no responses, it may change to another channel and repeat the process. At the conclusion of the inquiry, the station has accumulated information about the wireless LAN cells in its vicinity.
Joining a wireless LAN cell requires that all of the mobile station's MAC and physical parameters be synchronized with the desired wireless LAN cell. To do this, the station must update its timer with the value of the timer from the wireless LAN cell description, modified by adding the time elapsed since the description was acquired. This will synchronize the timer to the wireless LAN cell. Once this process is complete, the mobile station has joined the wireless LAN cell and is ready to begin communicating with the stations in the wireless LAN cell.
Each wireless station and access point in an IEEE 802.11 wireless LAN implements the MAC layer service, which provides the capability for wireless stations to exchange MAC frames. The MAC frame is a packet that transmits management, control, or data between wireless stations and access points. After a station forms the applicable MAC frame, the frame's bits are passed to the Physical Layer for transmission.
Before transmitting a frame, the MAC layer must first gain access to the network. Three interframe space (IFS) intervals defer an IEEE 802.11 station's access to the medium and provide various levels of priority. Each interval defines the duration between the end of the last symbol of the previous frame to the beginning of the first symbol of the next frame. The Short Interframe Space (SIFS) provides the highest priority level by allowing some frames to access the medium before others, such as an Acknowledgement (ACK) frame, a Clear-to-Send (CTS) frame, or a subsequent fragment burst of a previous data frame. These frames require expedited access to the network to minimize frame retransmissions.
The Priority Interframe Space (PIFS) is used for high priority access to the medium during the contention-free period. A point coordinator in the access point connected to the backbone network controls the priority-based Point Coordination Function (PCF) to dictate which stations in a cell can gain access to the medium. The point coordinator in the access point sends a contention-free poll frame to a station, granting the station permission to transmit a single frame to any destination. All other stations in the cell can only transmit during a contention-free period if the point coordinator grants them access to the medium. The end of the contention-free period is signaled by the contention-free end frame sent by the point coordinator, which occurs when time expires or when the point coordinator has no further frames to transmit and no stations to poll.
The distributed coordination function (DCF) Interframe Space (DIFS) is used for transmitting low priority data frames during the contention-based period. The DIFS spacing delays the transmission of lower priority frames to occur later than the priority-based transmission frames. An Extended Interframe Space (EIFS) goes beyond the time of a DIFS interval as a waiting period when a bad reception occurs. The EIFS interval provides enough time for the receiving station to send an acknowledgment (ACK) frame.
During the contention-based period, the distributed coordination function (DCF) uses the Carrier-Sense Multiple Access With Collision Avoidance (CSMA/CA) contention-based protocol, which is similar to IEEE 802.3 Ethernet. The CSMA/CA protocol minimizes the chance of collisions between stations sharing the medium by waiting a random backoff interval if the station's sensing mechanism indicates a busy medium. The period of time immediately following traffic on the medium is when the highest probability of collisions occurs, especially where there is high utilization. Once the medium is idle, CSMA/CA protocol causes each station to delay its transmission by a random backoff time, thereby minimizing the chance it will collide with those from other stations.
In the IEEE 802.11 Standard, the channel is shared by a centralized access protocol, the Point Coordination Function (PCF), which provides contention-free transfer based on a polling scheme controlled by the access point (AP) of a basic service set (BSS). The centralized access protocol gains control of the channel and maintains control for the entire contention-free period by waiting the shorter Priority Interframe Space (PIFS) interval between transmissions than the stations using the Distributed Coordination Function (DCF) access procedure. Following the end of the contention-free period, the DCF access procedure begins, with each station contending for access using the CSMA/CA method.
The 802.11 MAC Layer provides both contention and contention-free access to the shared wireless medium. The MAC Layer uses various MAC frame types to implement its functions of MAC management, control, and data transmission. Each station and access point on an 802.11 wireless LAN implements the MAC Layer service, which enables stations to exchange packets. The results of sensing the channel to determine whether the medium is busy or idle are sent to the MAC coordination function of the station. The MAC coordination also carries out a virtual carrier sense protocol based on reservation information found in the Duration Field of all frames. This information announces to all other stations the sending station's impending use of the medium. The MAC coordination monitors the Duration Field in all MAC frames and places this information in the station's Network Allocation Vector (NAV) if the value is greater than the current NAV value. The NAV operates similarly to a timer, starting with a value equal to the Duration Field of the last frame transmission sensed on the medium and counting down to zero. After the NAV reaches zero, the station can transmit if its physical sensing of the channel indicates a clear channel.
At the beginning of a contention-free period, the access point senses the medium; and if it is idle, it sends a beacon packet to all stations. The beacon packet contains the length of the contention-free interval. The MAC coordination in each member station places the length of the contention-free interval in the station's Network Allocation Vector (NAV), which prevents the station from taking control of the medium until the end of the contention-free period. During the contention-free period, the access point can send a polling message to a member station, enabling it to send a data packet to any other station in the BSS wireless cell.
Quality of service (QoS) is a measure of service quality provided to a customer. The primary measures of QoS are message loss, message delay, and network availability. Voice and video applications have the most rigorous delay and loss requirements. Interactive data applications such as Web browsing have less restrained delay and loss requirements, but they are sensitive to errors. Non-real-time applications such as file transfer, email, and data backup operate acceptably across a wide range of loss rates and delay. Some applications require a minimum amount of capacity to operate at all—for example, voice and video. Many network providers guarantee specific QoS and capacity levels through the use of Service-Level Agreements (SLAs). An SLA is a contract between an enterprise user and a network provider that specifies the capacity to be provided between points in the network that must be delivered with a specified QoS. If the network provider fails to meet the terms of the SLA, then the user may be entitled a refund. The SLA is typically offered by network providers for private line, frame relay, ATM, or Internet networks employed by enterprises.
The transmission of time-sensitive and data application traffic over a packet network imposes requirements on the delay or delay jitter, and the error rates realized; these parameters are referred to generically as the QoS (Quality of Service) parameters. Prioritized packet scheduling, preferential packet dropping, and bandwidth allocation are among the techniques available at the various nodes of the network, including access points, that enable packets from different applications to be treated differently, helping achieve the different quality of service objectives. The above-cited, copending U.S. patent application, entitled “Tiered Contention Multiple Access (TCMA): A Method for Priority-Based Shared Channel Access,” describes the Tiered Contention Multiple Access (TCMA) distributed medium access protocol that schedules transmission of different types of traffic based on their QoS service quality specifications.
For multiple-cell wireless LANs, the limited availability of channels implies that the channels must be re-used, much like in cellular communication networks. But unlike in cellular networks, the number of channels available in wireless LANs is not adequate to ensure both contiguous coverage (which is essential for roaming) and interference-free connections at the same time. As a result, cells assigned the same channel may experience co-channel interference in the area of overlapping coverage or near a cell's periphery. The problem of overlapping cell coverage is acute when wireless LANs are installed without any awareness of what other wireless LANs are operating nearby. Consequently, multiple-cell wireless LANs must rely on a medium access control (MAC) protocol to allocate channel time among stations in order to avoid co-channel interference between cells, just as it avoids contention among stations within the same cell.
Special MAC protocols are provided for wireless LANs because transmission is flawed by higher bit error rates, different losses are experienced on a wireless channel depending on the path on which the signal travels, and a radio node cannot listen while transmitting. Additive noise, path loss and multipath result in more retransmissions and necessitate acknowledgements, as successful transmission cannot be taken for granted. The different losses experienced along different paths cause different nodes to receive transmissions at different strengths, giving rise to the phenomenon of hidden terminals, as is known in the art. These are terminals that cannot hear or be heard by a source but are capable of causing interference to the destination of a transmission. The message exchange mechanism known in the art as Request-to-Send/Clear-to-Send (RTS/CTS) alleviates the hidden terminal problem. RTS/CTS also provides a reservation mechanism that can save bandwidth in wireless LANs. The inability to detect a collision as quickly as it can be detected on cable with carrier-sense multiple access with collision detection (CSMA/CD) causes more channel time to be wasted in a collision while waiting for the entire frame to transmit before the collision is detected. Hence, carrier sensing is combined with the RTS/CTS mechanism to give carrier-sense multiple access with collision avoidance (CSMA/CA).
All channel reservations, generated either with an RTS/CTS exchange or for a contention-free period (CFP), are made with the aid of the Network Allocation Vector (NAV), which is a timer maintained by all stations. The NAV is set at the value of the duration field broadcast when the reservation is announced, either by the RTS or CTS frames, or with the PCF beacon transmitted by the AP to initiate the CFP. All stations in a cell defer access until the NAV expires. The NAV thus provides a virtual carrier-sense mechanism.
Receiving signals at different strengths, depending on their origin, gives rise to capture effects. A known capture effect, the “near-far capture,” results from stronger signals being received successfully while other stations transmit at the same time. Near-far capture leads to inequities, as throughput is greater for nearby stations while distant stations are starved. In infrastructure wireless LANs, where all communications occur through the AP, the inequity can be remedied by applying power control at the station (i.e., on the uplink). By equalizing the signal strength received at the AP, all transmissions have equal probabilities of success.
A special IEEE 802.11 study group is working on enhancements to the MAC protocols that achieve acceptable QoS for Wireless LANs. Proposals for a QoS enhanced DCF (EDCF) mechanism and a QoS enhanced PCF (EPCF) mechanism are under review.
The proposed EDCF mechanism employs the Tiered Contention Multiple Access (TCMA) protocol. The basic access rules of TCMA are similar to CSMA with the following differences: transmission deferral and backoff countdown depend on the priority classification of the data. A station still waits for an idle time interval before attempting transmission following a busy period, but the length of this interval is no longer equal to DIFS. The length of an idle time interval is equal to the Arbitration-Time Inter-Frame Space (AIFS), which varies with the priority of the data. A shorter AIFS is associated with higher priority data. As a consequence, higher priority data gets to the channel faster. In addition, countdown of the backoff timer does not commence when a busy period completes unless the channel has been idle for a period equal to AIFS. This causes backoff countdown of lower priority frames to slow down and even freeze if there are higher-priority frames ready to transmit, a common occurrence in congestion.
The proposed EPCF maintains multiple traffic queues at the stations for different traffic categories. Higher-priority frames are scheduled for transmission first. Delays are reduced through improved polling-list management. Only active stations are kept on the polling list. A station with data to transmit must reserve a spot on that list, where it stays as long as it is active and for a limited number of inactive polling cycles. In the proposed draft standard, the reservation occurs inside the CFP, using a multi-channel ALOHA channel access mechanism to forward reservation requests. A priority mask is available to restrict contention by priority in case of congestion. Several of the features in EPCF are part of the MediaPlex protocol.
The hybrid coordination function (HCF) has been proposed to provide a generalization of PCF. It allows for contention-free transfers to occur as needed; not necessarily at pre-determined regular repeat times as provided by the PCF. The AP can thus send (and possibly receive) data to stations in its BSS on a contention-free basis. This contention-free session, referred to as a contention-free burst (CFB), helps an AP transmit its traffic, which is typically heavier in infrastructure cells (since stations must communicate exclusively through the AP). As in the case of the PCF, the HCF permits access to the channel by the AP after waiting for an idle period of length equal to PIFS.
Attention has also been given by the study group to the problem of co-channel overlapping BSSs (OBSSs). Channel re-use in multiple-cell Wireless LANs poses a problem for the PCF and HCF, as contention-free sessions (CFSs) are generated without coordination among co-channel APs to help prevent time overlap. Some mechanism is needed in situations where cells are within interference range of each other. The existing standard does not provide adequate coordination for contention-free sessions in such situations. The DCF mechanism does not require special measures, as stations operating under the DCF mechanism deal with interference from stations in other cells in exactly the same manner as they deal with interference from stations in their own cell.
All stations within the cell operate on one duplex TDD channel, with only one station in each cell transmitting data at any given time. In order to preserve power, stations go into a sleeping mode, which prevents frequent changes of the operating channel. Channel assignments should thus be fixed or static. Static assignments permit slow adaptation to traffic pattern changes over the course of a day. Ideally, these fixed or static assignments must be made optimal through the use of fixed or adaptive non-regular channel assignment methods, which are based on measurement-derived re-use criteria known in the art. With such an approach, statistical interference relationships between cells are established from measurements of the signal strength between stations and APs in different cells. Optimization methods use these relationships to assign the available channels to cells. Ad hoc channel assignment methods, like Dynamic Frequency Selection of HiperLAN2, can be used but with less promising results, as the re-use distances between co-channel cells are not selected optimally.
The limited number of channels available in the unlicensed band (three channels for IEEE 802.11b) will lead to a high degree of overlap in the coverage areas of co-channel cells. This overlap is exacerbated by the ad hoc placement of wireless LANs that results in overlapping BSAs. The channel time (or bandwidth) must thus be allocated among multiple co-channel cells in order to avoid interference. To be efficient, the channel should not remain idle if there is data waiting for transmission. Thus, while channel selection must be fixed or static, bandwidth allocation should be dynamic (possibly changing on a per-transmission basis).
A distributed dynamic bandwidth allocation mechanism is simply a distributed contention-based MAC protocol, which must enable sharing of the channel among APs and DCF stations in co-channel cells, as HCF and DCF co-exist. With APs accessing the channel to initiate contention-free sessions (CFPs or CFBs) before DCF stations, a prioritized distributed MAC protocol is needed. Such a protocol would also handle different priority DCF data.
The priority-based distributed MAC protocol for EDCF, TCMA, can be used to allocate the channel time among co-channel cells in a multiple-cell wireless LAN. The APs would be treated as a class with priority above the highest DCF priority class and would be assigned, therefore, a shorter AEFS than the highest-priority EDCF data. Other variations of CSMA are also appropriate.
In general, a carrier-sense-based MAC protocol would help avoid interference between cells as it causes conflicting transmissions—either DCF transmissions or CFSs—to occur at statistically (or deterministically, depending on the protocol) different times in co-channel cells.
The objective of dynamic bandwidth allocation is to promote fair access to the channel for all co-channel cells. That is, the success rate of a cell in accessing its assigned channel, either by its AP generating CFSs or by (E)DCF transmissions, should be independent of its location, assuming comparable traffic loads. Without fair access, transmissions can be delayed excessively in the disadvantaged cell, thus failing to meet QoS requirements. This goal is not realized with a traditional CSMA-type of protocol, however, when channel re-use is allowed because of a neighborhood capture effect.
Neighborhood capture arises when Ethernet-type protocols are employed in multiple-cell wireless LANs that re-use radio frequency (RF) channels. Given the small number of channels available, co-channel cells cannot all transmit simultaneously without causing interference on one another. A carrier-sense contention-based MAC protocol can allocate channel bandwidth among co-channel cells dynamically and in a distributed manner; but if used in the conventional way, it may lead to channel capture. Mutually non-interfering co-channel neighbors could deprive other co-channel neighbors of access. In general, there will be instability, with the channel retained by a group of cells for long time intervals. This would have negative impact on quality of service (QoS).
Neighborhood capture arises in a multiple-cell wireless LAN with fewer channels available than the number of cells. Unlike in cellular communications networks, where sufficient channels are available to ensure interference-free transmission on an assigned channel, channel selection in WLAN networks must be accompanied by dynamic bandwidth allocation in order to avoid interference between co-channel cells.
Carrier-sense multiple access (CSMA)-type media access control (MAC) protocols provide dynamic bandwidth allocation in a distributed manner, obviating the need for a central controller. With such protocols, time-overlapped transmissions by stations in non-interfering co-channel cells cooperate to capture the channel for long time periods. The resulting neighborhood capture is deleterious to QoS because of the ensuing access delays in other co-channel cells.
The present invention addresses neighborhood capture and establishes a method to prevent its occurrence.